EPR investigation of metal oxide catalysts: ferromagnetic behaviour induced by oxygen vacancies
Pure and doped metal oxides are widely used for many catalytic applications. This work mainly focuses on In2O3-based catalysts for CO2 conversion to methanol through hydrogenation. The catalytic activity of these materials is thought to be related to the presence of oxygen vacancies, which act as catalytic centers. Combining In2O3 with ZrO2 support and doping it with Pd provides increased stability against sintering and H2 splitting ability, respectively.[1-2] Moreover, ZrO2 and Pd ions could also be involved in vacancies creation and annihilation and could thus be crucial for the catalytic mechanism.
It is widely known that oxygen vacancies can trap unpaired electrons, thus becoming paramagnetic, and can therefore be studied by EPR. Isolated paramagnetic vacancies (also called color centers) give rise to sharp, usually almost isotropic signals with g factors near to ge.[3] However, as we show here, when the density of vacancies is very high, unexpected broad signals appear, with features characteristic of ferromagnetic systems. These signals can be attributed to exchange-coupled magnetic polarons. The detailed EPR investigation of these signals provides important information about the structure and the dynamics of different kind of vacancies. Moreover, a quantitative approach to ferromagnetic signals and EPR-detected hysteresis phenomena is proposed.
The importance of these findings is related not only to metal oxide catalysis, but also to the fundamental understanding of the magnetic properties of metal oxides and provide a methodology to produce room temperature ferromagnets in a controlled fashion.